In a Chihuahuan desert ecosytem, the growth responses to nitrogen amendment of a perennial shrub (creosotebush, Larrea tridentata) and a perennial grass (fluff grass, Erioneuron pulchellum) were made twice during the growing season: first during a period of average rainfall, then during a period of above average rainfall. Fluff grass exhibited a marked increase in biomass to 25 kg ha-1 and 100 kg ha-1 nitrogen fertilization. Creosotebush increased biomass production only at the higher level during both periods. These results suggest that productivity of a Chihuahuan desert system is partially nitrogen limited and that shallow-rooted plants utilized the available nitrogen at the 25 kg ha-1 fertilization level, thus making it unavailable to creosotebush.